Reservoirs of surface water are rarely as clean as groundwater. To reproduce the natural cleansing of aquifers, reservoir operators must use treatment devices like these antique sand filters at Chabot Reservoir.

Wherever settlers arrived in America, their first concern was water. The ideal source of good water was not a babbling brook, although that was good enough for irrigating crops and brewing beer. Surface water varies with the seasons, is readily muddied, and we all know what fish do in it. Best was a steady, cool flow from a protected spring—that is, groundwater. Before wellsprings were a metaphor, they were the very basis of America’s first settlements. (March 6–12, 2011 is National Groundwater Awareness Week to help remind us of these historic truths.)

As settlements grew into towns that in turn became cities, larger water sources could be engineered. Ancient Rome owed its prominence to its abundant water supply, brought by aqueduct from Apennine Mountain springs. Mighty New York captured the clean Catskill rains in a network of reservoirs that fed their water by gravity to Manhattan. San Francisco raided a national park in the Sierra Nevada, a hundred miles away, for its water supply. Now every Bay Area river has been harnessed for civic purposes. But groundwater is still a major player here, if a largely invisible one.

Groundwater wells were part of every household in the early days, but the typical shallow aquifer, or water-bearing zone, is not suited for hard use. It’s too closely connected to the surface; indeed every permanent stream can be thought of as living groundwater, where erosion cuts into the top of the aquifer. Once too many wells tap the surface aquifer, the level of water underground—the water table—is depressed. Eventually the streams are affected, and the costs of digging ever deeper bring an end to the household well.

A growing city has to be more organized about its water. Most Bay Area cities rely on agencies that deliver surface water from a reservoir. The big advantages of a reservoir are energy and size: it’s easy to deliver water downhill from the dam, and centralized treatment plants can do an efficient job filtering and disinfecting the water. The big advantages of groundwater are its quality, its closeness and its resistance to drought. Both types of water source must be carefully managed for the long term by well-trained technicians.

Today the South Bay still relies on groundwater. San Jose pumps about 40 percent of its water from aquifers beneath it. Nearby Sunnyvale, Campbell and Santa Clara also produce significant amounts. Farther south, Morgan Hill and Gilroy rely exclusively on groundwater. And Fremont has a big stake in it too.

San Jose used to pump more aggressively than today, but problems arose when the land began to sink. Parts of Alviso, on the city’s northern edge, recorded as much as 13 feet of subsidence. Seawater began to intrude into the aquifers as well. Today water managers have arrested the subsidence by ensuring that the aquifer is properly recharged using streamflow and special infiltration basins.

Much of Fremont’s water comes from its bountiful aquifer in the Niles Cone, a large fan of gravel spreading out from the mouth of Alameda Canyon. Today the city sends Alameda Creek’s water into the Cone through the Quarry Lakes, while pumping groundwater out of it in a strategy that helps push back invading Bay water.

San Francisco, of course, gets pristine Sierra water from O’Shaunnessy Dam in the Hetch Hetchy Valley. Only one-third of Hetch Hetchy’s water gets to the city, though, as more than a dozen other Bay Area cities use it.

Hetch Hetchy Aqueduct crosses the Great Valley near route 132, marked by monuments like this.

Although large-scale water projects serve the masses, there is still a place for the custom well. Outlying residents, farms and landscape-intensive businesses can often save money using water from their own property. Today these uses of groundwater provide our region with much-needed resilience in the face of drought and earthquake. Wherever we live, the role of groundwater continues—just below the surface.

Author

Andrew Alden

Andrew Alden earned his geology degree at the University of New Hampshire and moved back to the Bay Area to work at the U.S. Geological Survey for six years. He has written on geology for About.com since its founding in 1997. In 2007, he started the Oakland Geology blog, which won recognition as "Best of the East Bay" from the East Bay Express in 2010. In writing about geology in the Bay Area and surroundings, he hopes to share some of the useful and pleasurable insights that geologists give us—not just facts about the deep past, but an attitude that might be called the deep present.

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